1. Field of the Invention
The present invention relates to particle monitors. In particular, the present invention relates to dark-field particle monitors used in a manufacturing process.
2. Discussion of the Related Art
Dark-field particle monitors or sensors are often used in semiconductor processing equipment to monitor particle levels, which are critical to process yields. In a dark-field particle monitor or sensor, a particle passing through a collimated or focussed laser beam scatters laser light to a photocell located outside of the laser beam's direction of propagation. An example of such a particle sensor is described in U.S. Pat. No. 5,266,798 to P.Borden et al, entitled "High Sensitivity, Large Detection Area Particle Sensor for Vacuum Applications", Ser. No. 07/742,798, issued on Nov. 30, 1993, and filed on Aug. 8, 1991.
The performance of a dark-field sensor can be limited by a phenomenon known as stray or "DC" light. Stray light occurs when, as frequently happens, a sensor accumulates over time a coating or a film of dirt on the optical interfaces through which the laser beam passes. Naturally, this film of dirt scatters light ("stray light") from the laser beam. Because of the high intensity of the laser beam, the intensity of the stray light can be fairly high. Such stray light often illuminates the interior of the sensor housing, so that a portion of the stray light can find its way to the photodetector of the particle monitor.
Stray light limits the performance of the particle monitor in a number of ways. First, stray light is modulated by noise in the laser, so that strong stray light introduces the laser noise into the photodetector circuit, thereby causing inaccuracy in the particle count.
Second, although stray light intensity is normally relatively constant, so that the "DC" portion (i.e. low varying portion) of stray light intensity can be subtracted from the laser intensity detected by the particle monitor, a vibration in the particle detectors induces an "AC" component (i.e. high varying portion) in the intensity of stray light. This "AC" component results from stray light reflected off vibrating elements in the particle monitor. Particle monitors are highly sensitive to such an "AC" component, resulting in an erroneous particle count.
Third, stray light induces shot noise in the photodetectors of the particle monitor. Shot noise results from the statistical variation in the arrival rate of photons. The current I.sub.noise, which is a noise current induced by stray light in a photodetector, is given by: EQU I.sub.noise =.sqroot.2qBP.sub.stray A
where q is the charge of an electron, B is the bandwidth of the photodetector, P.sub.stray is the power of the incident stray light, and A is the conversion efficiency of the photodetector. The I.sub.noise current limits the signal-to-noise ratio of the photodetector, thereby limiting the photodetector's sensitivity to small particles.
For the above reasons, reduction in the incident intensity of stray light in particle monitors is highly desirable.